When building a marine oil production installation it is necessary to lay a pipe or a pipeline for the transportation of oil, gas or another fluid from an installation, preferably a satellite field, to another installation, preferably a processing platform. The pipe or the pipeline may consist of two or more pipe sections which are welded together to form a continuous pipe which is laid on the sea floor and subsequently lowered into a trench ploughed therein. After some time the trench sands up and the pipe is typically covered with between one half to one meter of sand. In case of oil or gas from a satellite platform, the fluid is often warm and therefore heats the covered pipeline. If the pipeline has a length of more than a few hundred meters, the pipe is retained by the surrounding area sea bed by means of friction and therefore it cannot expand as a result of the increase in temperature in the pipe wall. Instead, compressive stress develops longitudinally in the pipe wall. The pipeline from a satellite platform is usually dimensioned to resist the full pressure at the well head. The pressure may typically be approximately 200 bar. If such a pipe is heated to a temperature of 75.degree. C. above the temperature of the surroundings, considerable forces develop, for instance in a pipe with a diameter of 325 mm and a wall thickness of 20 mm a compressive force of 300,000 kp develops longitudinally. If the pipe could expand freely, this would correspond to an increase in length of 7.5 m over 10 km. However, as the pipe cannot expand freely, the heating causes the pipe to be unstable and to arch, i.e. to be deformed into an arch or an elbow above the sea floor, thereby damaging the pipeline. The above effect resulting from the temperature of the fluid is also enhanced by the pressure of the fluid in question. In case of a temperature difference of the above order, the influence of the pressure on the pipe is usually of lesser importance than that of the temperature. Therefore, only the temperature is mentioned below as the real source of the problem in question.
In order to prevent the above instability, the pipe may either be ploughed deeper into the sea bed or be covered by a heap of stones thereby preventing the pipe from arching by means of the superjacent sand or stones. Especially the latter method is very costly and time-consuming. Thus, it may be necessary to dump several ship loads of stones for each kilometer of pipeline (e.g. approximately 15 tons each meter). Furthermore, great demands are made on the straightness after the trenching, as small deviations increase the tendency of the pipeline to arch.
It is known from NO-B 136,475 to lay a pipe in a ditch in the ground and to provide said pipe with a predetermined prestressing by heating the pipe from the inside by means of a hot medium before said pipe is covered with earth. Such a method cannot, however, be used for conventional non-insulated pipes to be layed on the sea floor because the lengths of said pipes prevent a heating to the necessary temperatures before said pipes are covered with earth. When the pipe is insulated, the necessary heating can only be ensured at extreme costs.
DE-C2 29 09 475 discloses inter alia a use of double pipelines comprising an inner and an outer pipe, where said pipes are fixed relative to one another once the inner pipe has been prestressed relative to the outer pipe. Usually, such pipes are not used for laying on the sea floor, the latter probably being due to the fact that such pipes are both expensive and difficult to manufacture, and in addition the laying thereof is rather time-consuming because the inner and the outer pipes must be continuously welded together during the laying procedure from the vessel. Furthermore, the outer pipe must necessarily be of a rather thick material in order to withstand the effect of the inner pipe.
Gas Journal-Transmission Distribution, Vol 333, No. 5438, Jan. 17, 1968, "Prestressing pipelines", p 68 to 70, discloses a method of laying a double pipeline under water. When such a pipeline is used for transportation of heated fluids, the inner pipe is prestressed and fixed relative to the outer pipe. Such a pipeline is, however, encumbered with the same instability problems as the known single pipelines layed without any prestressing.